The present invention relates to an integrated circuit semiconductor device and more particularly, to a dynamic memory device having a plurality of memory cells each of which is constituted by a single transistor and a single capacitor coupled to the transistor.
A silicon nitride layer or a composite layer including a silicon nitride film and a silicon oxide film is used as a dielectric film of a MOS type capacitor because the silicon nitride has the high dielectric constant. On the other hand, a gate insulating film of an insulated gate field effect transistor (hereinafter abbreviated as IGFET) is made of thermally grown silicon oxide (SiO.sub.2) because of its highly reliable nature. However, in the prior art, a neck, that is, concave is inevitably caused in the silicon oxide film thermally grown as the gate insulating film of IGFET in the vicinity of the boundary between the silicon oxide film and the silicon nitride film as the dielectric film of the capacitor. Therefore, even if the silicon oxide film is constructed such that it extends along the bottom of the silicon nitride film at a distance of 150 .ANG. to 300 .ANG. (angstrom) from the boundary, that is, from the edge of the silicon nitride film, the breakdown voltage between the gate electrode and the substrate becomes about 15 V which is lower value than a desirable value of 30 V when the silicon oxide film has the thickness of 400 .ANG..